SARA-G3 series - System Integration Manual
UBX-13000995 - R06 Objective Specification Design-in
Page 101 of 218
Coupling between VCC and audio lines (especially microphone inputs) must be avoided, because the
typical GSM burst has a periodic nature of approx. 217 Hz, which lies in the audible audio range.
The tank bypass capacitor with low ESR for current spikes smoothing described in Figure 30Figure 30
and Table 16Table 16 should be placed close to the VCC pins. If the main DC source is a switching
DC-DC converter, place the large capacitor close to the DC-DC output and minimize the VCC track
length. Otherwise consider using separate capacitors for DC-DC converter and SARA-G3 module tank
capacitor.
The bypass capacitors in the pF range described in Figure 30Figure 30 and Table 16Table 16 should
be placed as close as possible to the VCC pins. This is highly recommended if the application device
integrates an internal antenna.
Since VCC is directly connected to RF Power Amplifiers, voltage ripple at high frequency may result in
unwanted spurious modulation of transmitter RF signal. This is more likely to happen with switching
DC-DC converters, in which case it is better to select the highest operating frequency for the switcher
and add a large L-C filter before connecting to the SARA-G3 series modules in the worst case.
If VCC is protected by transient voltage suppressor to ensure that the voltage maximum ratings are
not exceeded, place the protecting device along the path from the DC source toward the SARA-G3
module, preferably closer to the DC source (otherwise protection functionality may be compromised).
2.1.1.9 Guidelines for grounding layout design
Good connection of the module GND pins with application board solid ground layer is required for correct
RF performance. It significantly reduces EMC issues and provides a thermal heat sink for the module.
Connect each GND pin with application board solid GND layer. It is strongly recommended that each
GND pad surrounding VCC pins have one or more dedicated via down to the application board solid
ground layer.
The VCC supply current flows back to main DC source through GND as ground current: provide
adequate return path with suitable uninterrupted ground plane to main DC source.
If the application board is a multilayer PCB, then it is required to connect together each GND area
with complete via stack down to main board ground layer.
It is recommended to implement one layer of the application board as ground plane as wide as
possible.
Good grounding of GND pads also ensures thermal heat sink. This is critical during call connection,
when the real network commands the module to transmit at maximum power: proper grounding helps
prevent module overheating.
2.1.2 RTC supply (V_BCKP)
2.1.2.1 Guidelines for V_BCKP circuit design
If RTC timing is required to run for a time interval of T [s] at 25 °C when VCC supply is removed,
place a capacitor with a nominal capacitance of C [µF] at the V_BCKP pin. Choose the capacitor using
the following formula:
C [µF] = (Current_Consumption [µA] x T [s]) / Voltage_Drop [V]
= 1.5 x T [s]
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